The Yankee paper machine differs from the multi-cylinder paper machine in that the common Yankee machine has only one large drying cylinder with the aid of which the paper is dried. The web travels around this cylinder tightly adhered to its heated outer surface, extending around nearly the entire circumference of the cylinder, during which it usually dries completely. The diameter of this cylinder can be in the order of 5-6 meters, while the cylinders of the multi-cylinder paper machine commonly are 1.5 or 1.8 meters in diameter. As the web is adhered to the hot Yankee cylinder surface continuously during drying for a circumferential distance of from 10 to 15 meters without being loosened from the cylinder surface, this kind of paper making process and the paper made thereby differs considerably from the paper making on a multi-cylinder paper machine.
It is also possible to build paper machines in which a Yankee dryer is combined with a number of standard dryers in different ways.
One grade of paper made on a Yankee paper machine is the so-called "machine glazed" or MG paper which is characterized by having a high glaze on one side, the other side being more or less coarse.
Currently, Yankee paper machines are usually used for making creped papers. The most widely used and most important group of the creped papers is the so-called creped tissue grades, for example, facial tissue, napkin tissue, toilet tissue and toweling tissue.
The term creping refers to a process by which certain characteristics of paper are being improved such as, for example, the softness, absorbing capacity, stretch, and the like. The creping is produced by doctoring the paper web off the Yankee cylinder surface with the aid of a doctor blade. When the web runs against the doctor blade, it becomes creped, which means that horizontal wrinkles or waves are formed in the web transverse to the direction of travel of the web.
The adhesion of the web onto the cylinder surface has a great influence on the quality of the paper manufactured, whether it concerns the glaze of MG paper or the crepe properties of the tissue.
A successful creping process demands a web which has a proper adhesion to the cylinder surface at the position of the creping process. If the web is too weakly adhered, it can be partly loosened before the actual creping process. This creates an uneven and rough creping pattern which results in an uneven and low quality product. If the adhesion is too strong, it results in holes in the paper and even breaks on the paper machine.
The degree of adhesion of the wet web to the Yankee cylinder surface depends largely on the moisture content of the web at the moment when it is pressed against the surface of the cylinder. An exact figure for this moisture content cannot be given, because it depends very much on the fiber material used, on the temperature of the Yankee cylinder, on the pressing forces used, etc. In any case, it is clear that the mositure content of the web must be controllable so that the optimal adhesion can be reached.
Also, in manufacturing the MG paper it is desirable to optimize web moisture at the point where the web is pressed against the Yankee cylinder in order to achieve a good glaze for the paper.
In Yankee paper machines previously known, an arrangement has usually been applied wherein the web is detached from the forming wire with the help of a pick-up roll, which operates inside the so-called Yankee felt loop. The web, attached to the lower surface of this Yankee felt, travels then to the first press nip which is formed between the Yankee cylinder and the first press roll. As the web is led directly from the wire to the first nip which is formed against the Yankee cylinder, it is clear that the mositure content of the web at this nip depends solely on the dewatering capacity of the wire section of the paper machine. This drainage capacity in turn depends, for example, on the speed of the machine, basic weight of the web, properties and type of the stock, and other factors.
At present, as efforts are made to operate the machines at higher speeds and as new raw materials are used, such as de-inked waste paper fiber instead of or in addition to conventional chemical pulp and ground wood fibers, it often happens that the mositure content of the web is too high as it enters the first press nip. Therefore, it has become necessary to arrange a separate dewatering press nip between the wire section and the Yankee cylinder before the first Yankee press. This so-called wet press consists of a press roll inside the Yankee felt loop and a lower press roll pressed against it. The lower press roll can operate either without a felt or wrapped by a felt loop of its own. In both cases, it can happen at the wet press that the web does not follow the Yankee felt but drops off, and this causes a break in the machine operation.